The prior art hydraulic circuit for a hydraulically operated vehicle which uses a variable delivery hydraulic pump and a motor and the speed of which can be varied continuously is shown in FIG. 1, where a part of the output from an engine 5 drives a hydraulic pump 4 installed on a working machine. This machine further includes a hydraulic circuit 51 and a hydraulic cylinder 52. The engine output acting on the pump 4 is transmitted to the cylinder 52 via the hydraulic circuit 51. The remaining portion of the output from the engine 5 drives a control pump 3 and a hydraulic pump 1. The pressurized oil produced by the hydraulic pump 1 is forced into a variable delivery hydraulic motor 2 via main circuits 24 and 25 to rotate the motor 2, thereby driving the vehicle. The capacity of the hydraulic pump 1 is controlled by a valve 6 and a cylinder 7. Also shown are main relief valves 8, a charge relief valve 9, and a filter 10. Pressurized oil delivered from the valve 6 is passed through a motor control oil passage 11 and guided to one end of a motor control valve 14. Oil which is at the higher pressure is forced through a pilot pipe 16 from the main circuits 24 and 25 and then into a cylinder 31 that controls the capacity of the motor 2.
That is, the pump control valve 6 and the motor control valve 14 control the cylinders 7 and 31, respectively, to arbitrarily vary the capacities of the hydraulic pump 1 and the hydraulic motor 2, respectively, for changing the speed of the vehicle.
In this nonstep variable speed, hydraulically operated vehicle using the prior art variable delivery hydraulic motor, the driving power and the speed of the vehicle can be varied continuously. The speed can be automatically changed from 0, i.e., the maximum driving power, to the maximum speed without the need to manually vary the speed. Since the operator can control both vehicle speed and driving power only with the accelerator pedal, the operation is easier to perform than a vehicle equipped with a mechanical transmission.
On the other hand, in an industrial vehicle having a lifting implement such as a shovel loader, the pump of the lifting implement such as the pump 4 is driven by an engine such as the engine 5 in the same way as the hydraulic pump 1. The maximum speed of ascent of the lifting implement is proportional to the engine speed and so the speed of ascent of the lifting implement is controlled according to the position of the accelerator pedal in the same way as the vehicle speed.
Accordingly, where the vehicle moves while elevating the lifting implement in a narrow working site in which the vehicle travels a short distance, if the lifting implement is elevated at the maximum speed while moving the vehicle at a low speed, then the working efficiency is high. In the prior art techniques, however, if the lifting implement is elevated at the maximum speed, it is inevitable that the vehicle speed is increased to its maximum speed.
One conventional method for eliminating such a conflicting relation between the vehicle speed and the ascending speed of the lifting implement is to keep the motor capacity at the maximum capacity while suppressing the vehicle speed. Another conventional method is to connect a mechanical transmission with the motor, for switching the speed between several discrete values. Where the former method is adopted, the motor capacity is fixed and, therefore, the vehicle speed is limited to low values. Where the latter method is employed, the maximum driving power cannot be obtained within a high range. Therefore, it is impossible to cope with the operating conditions accurately.